Complementary metal oxide semiconductor (CMOS) image sensors are gaining in popularity over traditional charged-coupled devices (CCDs) due to certain advantages inherent in the CMOS image sensors. In particular, CMOS image sensors typically require lower voltages, consume less power, enable random access to image data, may be fabricated with compatible CMOS processes, and enable integrated single-chip cameras.
CMOS image sensors utilize light-sensitive CMOS circuitry to convert light energy into electrical energy. The light-sensitive CMOS circuitry typically comprises a photodiode formed in a silicon substrate. As the photodiode is exposed to light, an electrical charge is induced in the photodiode. The photodiode is typically coupled to a MOS switching transistor, which is used to sample the charge of the photodiode. Colors may be determined by placing filters over the light-sensitive CMOS circuitry.
The light received by pixels of the CMOS image sensor is often based on the three primary colors: red, green, and blue (R, G, B), and additional colors can be identified and/or created with various combinations and intensities (e.g., when red and green overlap they form yellow). Pixel sensitivity for receiving the incident light, however, is lowered with the trend of size reduction of pixels of the CMOS image sensor and cross-talk is caused between different pixels against incident light, especially against incident light with a long wavelength such as the red light (wavelength of about 650 nm), thereby degrading the overall performance of pixels of the CMOS image sensor.
As is known, image sensors can be designed to be illuminated from a front surface or from a back surface. Backside illumination image sensors provide an advantageous feature wherein the need to carefully place and route metallization features so as not to interfere with the optical path is eliminated, because the illumination comes from the backside of the wafer, whereas metallization is formed on the front side of the wafer. Additionally, the overall optical path, i.e., the optical distance from the focusing lens of the sensor to the actual light receiving surface of the sensor itself is typically reduced with backside illumination, relative to front-side illumination because the light need not travel through the metallization and inter-metal dielectric layers.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the embodiments and are not necessarily drawn to scale.